In recent years, intravascular ultrasound (IVUS) imaging systems have been developed for use in the diagnosis and treatment of cardiovascular and peripheral vascular disease. Such systems generally have one or more miniaturized transducers mounted on the tip of a catheter to provide electronic signals to an external imaging system in order to produce an image of the lumen of the artery or other vessel into which the catheter is inserted, the tissue of the vessel, and/or the tissue surrounding the vessel. These systems provide important diagnostic information which is not available from other more conventional techniques such as x-ray angiography. This information includes the location, amount and composition of arteriosclerotic plaque, and enables physicians to identify lesion characteristics, select an optimum course of treatment, position therapeutic devices and promptly assess the results of treatment.
Transluminal angioplasty utilizes an inflatable balloon at the distal end of an elongated flexible catheter to eliminate a blockage, or stenosis, produced by an accumulation of fatty tissue, or plaque, on the inner wall of a blood vessel or artery. The catheter is inserted into the vascular system and advanced along a guide wire to position the balloon next to the stenosis. When the balloon is properly positioned, it is inflated with pressurized fluid to compress the plaque and thereby relieve the stenosis.
The location of the stenosis and the positioning of the balloon were originally determined by techniques such as x-ray angiography and fluoroscopy wherein a radiopaque dye is injected into the vessel and radiopaque markers are mounted on the balloon, and x-ray imaging is employed to determine the location of the narrowing and the position of the balloon.
More recently, ultrasonic imaging has been used in combination with angioplasty in order to provide more detailed information about the stenosis. In one such technique, two separate catheters are employed, one having an ultrasonic imaging device at it distal end, the other having an inflatable balloon at its distal end. The balloon catheter is inserted along a guide wire and positioned by conventional techniques, and the balloon is inflated to treat the stenosis. That catheter is then removed, and the imaging catheter is inserted to enable the physician to examine the stenosis and determine if further treatment is needed. This technique does provide more information about the stenosis, but it requires an exchange of catheters and does not permit real time imaging of the dilation procedure.
Heretofore, there have also been some attempts to combine an ultrasonic imaging device and an inflatable balloon on a single catheter. With this approach, the imaging transducer can be used in the positioning of the balloon, the need to exchange catheters is eliminated, and the transducer can provide real time imaging of the dilation procedure, as well as images of the affected region after the treatment.
One problem with having the transducer and the balloon on a single catheter is that the transducer, being larger in profile than the deflated balloon, can prevent the balloon from being deployed in narrower stenosis. This is particularly so when the transducer is positioned distally of the balloon.
U.S. Pat. No. 5,167,233 describes a catheter in which the transducer is positioned proximally of the balloon. This permits the balloon to be inserted into stenoses which would be too narrow to be treated if the transducer had to pass through them first. However, having the transducer proximal to the balloon does present a problem in that the pressurized fluid must flow past the transducer, both during inflation and during deflation of the balloon. If the flow path is unduly restricted, inflation and deflation of the balloon will be undesirably slow.